An airship or dirigible is a type of aerostat or lighter-than-air aircraft that can navigate through the air under its own power. Aerostats gain their lift from large gas bags filled with a lifting gas that is less dense than the surrounding air. The envelope of an airship may form a single gas bag, or may contain a number of internal gas-filled cells. An airship also has engines, crew, and optionally also payload accommodation, typically housed in one or more "gondolas" suspended below the envelope.